Spherical Bessel Functions

Dear support team,

I whish to add spherical Bessel functions to my basis in SIESTA, but have not found how to.
From Siesta’s documentation, one can have a spherical Bessel function if ‘a species with atomic number = -100’ is defined. This raises several questions :
(1) we thus need to define this species, let us call it X, in the ChemicalSpeciesLabel block. Do we need a pseudopotential file for X ? If so, what is it ?
(2) we should also need to define the position of X in the AtomicCoordinatesAndAtomicSpecies block. However, we do not use X as a ghost atom to be put at a different location than the ‘real/actual’ atoms, but we need to put X at the exact same place as a real atom. Is this possible ?
(3) What is the behavior of X when we relax the atomic positions ? Is it taken as an actual atom, or does it follow the real atom it is placed upon ?

Thank you for your time.